Imaging, communication, and spectroscopic applications in the mid- and far-infrared regions have underscored the importance of developing reliable sources and detectors operating in the frequency range from 0.3 to 10 THz (1000 to 30 μm wavelength). Recent studies, such as T-Ray Imaging by D. Mittleman et al. in IEEE Journal of Selected Topics in Quantum Electronics vol. 2 1996 and TeraHertz Technology by P. Siegel in IEEE Transactions on Microwave Theory and Techniques vol. 50 2002, suggest that terahertz interactions can enable a variety of new applications on a wide range of solids, liquids, gases, including polymers and biological materials such as proteins and tissues.
For example, the resonant frequencies of many rotational and stretching transitions in complex organic molecules, such as proteins, are in this frequency range. Also, phonon energies of polar molecules may be in this range. Thus, THz frequency radiation sources may find significant uses in the fields of spectroscopic analysis and/or photochemical processes involving these molecules.
Additionally, many materials are relatively transparent to THz frequency radiation, making a number of imaging applications are possible. This allows THz frequency radiation to be used to create images similar to X-rays. As the photon energy of THz frequency radiation is significantly less than that of X-rays, THz frequency radiation, or T-ray, images may be made without the ionizing radiation associated with X-ray images. Thus, T-ray images may present fewer potential risks to the patient than present X-ray images.
Compared to microwave devices, devices operating in the THz, or far-infrared, frequency range may allow significant reductions in antenna size, as well as providing greater communication bandwidth. Additionally, the shorter wavelength of THz frequency waves, compared to microwaves, allows greater resolution with THz frequency waves than is possible with microwaves. Commercial applications may include thermal imaging, remote chemical sensing, molecular spectroscopy, medical diagnosis, fire and combustion control, surveillance, and vehicle driver vision enhancement. Military applications may include night vision, rifle sight enhancement, missile tracking, space-based surveillance, and target recognition.
Quantum cascade lasers (QCL's) fabricated from III-V materials have demonstrated light emission at wavelengths typically shorter than 10 μm over large temperature ranges. However, these III-V compound semiconductor devices may have limitations due to the strong reststrahlen phonon absorption at THz frequencies. THz frequency radiation has also reported from silicon-based quantum well structures by G. Dehlinger et al. in Intersubband Electroluminescence from Silicon-Based Quantum Cascade Structures, Science, vol. 290, Dec. 22, 2000. As with the III-V material QCL's, these silicon-based quantum cascade devices require the use of carefully controlled, epitaxial-processing techniques to form quantum well structures with sub-nanometer dimensional tolerances.
Optically-pumped, phosphorus-doped silicon THz emitters have also been reported in Stimulated Emission from Donor Transitions in Silicon, Physical Review Letters 84 (2000) by S. Pavlov et al. These optically-pumped emitters suffer from a small absorption cross-section for the CO2 laser radiation (10.6 μm wavelength) used to pump the devices.